1. Field of the Invention
The present invention relates to a process for preparing a perovskite type superconductor film, particularly Bi-(Pb)-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems with a high content of the high Tc phase, i.e., having a high critical temperature Tc of superconductivity.
2. Description of The Related Art
The research and development of high temperature superconductive materials exhibiting superconductivity at above the boiling point of liquid nitrogen is proceeding rapidly, and investigations into practical applications thereof are also underway. Particularly, superconductors having a critical temperature Tc above 100K are under investigation because they advantageously have a large temperature margin for liquid nitrogen application, theoretically have an increased critical current density Jc and critical magnetic field Hc.
Known superconductors having a critical temperature Tc above 100K are Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems. These Bi- and Tl- systems have the advantages of a resistance to water and oxygen degradation. Nevertheless, the Bi- and Tl-system superconductors have a disadvantage in that a low Tc phase is easily formed and the formation of single high Tc phase is difficult. The Bi-system is typically a mixed phase of a low Tc phase with a critical temperature Tc of about 80K and a high Tc phase with a Tc of about 110K. The Tl-system is typically a mixed phase of a low Tc phase with a critical temperature Tc of about 105K and a high Tc phase with a Tc of about 125K. The formation of isolated high Tc phase has not been accomplished.
A new high Tc Bi-Sr-Ca-Cu-O system superconductor was found by Maeda et al (Jpn. J. Appl. Phys. 27, 1988, L209). It was also found that this system contains three superconducting phases represented by the formula Bi.sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.x. A first phase corresponding to n=1 in the formula and having a Tc of 7K, a second phase corresponding to n=2 and having a Tc of 80K, and a third phase corresponding to n=3 and having a Tc of 105K. With the increase of n from 1 to 3, the number of CuO layers in the crystal structure is increased from 1 to 3 and the c axis of the crystal is elongated from 2.4 nm to 3.0 nm to 3.7 nm.
Takano et al reported that the volume fraction of the high Tc phase is increased by partially substituting Pb for Bi (Jpn. J. Appl. Phys. 27, 1988, L1041), but the role of Pb is not clear at present.
In a typical process for preparing a superconductor film of, for example, a Bi-Sr-Ca-Cu-O system superconductor, an oxide is deposited on a substrate of MgO, SrTiO.sub.3, etc. by sputtering, evaporation, etc., followed by a heat treatment to react the deposited film, to thereby form a superconducting film. There is a need to provide a process for reproduciblly forming a single phase film, having a high Tc phase, of the Bi-(Pb)-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems.